The use of microstrip lines with air as the dielectric has been used in base station antennas since at least the mid-90s. The advantage is low loss and ease of manufacture in the sense that a flat ground plane can be used for the lines. The disadvantage of such lines is the necessary width, which for example at a 3 mm separation is 13 mm for a 50 ohms line. The absence of a dielectric, other than air, also means that the mutual coupling is rather high.
As another prior art, in respect of the present invention, U.S. Pat. No. 5,652,557 could be mentioned. This patent discloses a transmission line consisting of a longitudinal groove in a dielectric material having longitudinal walls and bottom disposed with a metallisation film. In the groove, a conductor line is formed by a metal film.
However, the conductor of U.S. Pat. No. 5,652,557 is manufactured by disposing a conductor film on a dielectric. Such a production method can be complex and expensive, yielding an expensive transmission line.